In general, piezoelectric generators of mechanical vibrations using acoustic frequencies extending into the ultrasonic region are the basis of piezoelectric motors. In such motors, the operating frequency of the generator (and the output power of the motor) is controlled using standard pulsed current circuits. The generator is typically part of a sound-proof stator and assembled with a suitable mechanical transmission system to drive the rotor. In general, such generators are made in the form of a flat three-layer ring. The middle layer is typically a piezoelectric core, which is generally polarized at right angles with respect to its two planar surfaces. The upper and lower layers on either side of the core are typically made from electrically conductive coatings which act as electrodes and to which are attached conductors for supplying current pulses from a current source external to the motor. When electric current pulses are applied to the electrodes, vibrations in the piezoelectric core are set-up in the radial direction, causing the radius of the core to vary at one of its natural vibration frequencies.
In such piezoelectric motors, the efficiency typically depends on two factors: (1) the suitability of the piezoelectric generator for operating in resonance mode over a prolonged period of time; and (2) the ratio of energy stored in the system to that dissipated per cycle of oscillation, i.e., the quality factor (Q-factor) of the resonant system. With respect to (1), it is generally acknowledged that acoustic resonant vibrations in solid state materials can lead to a break-up of the materials depending upon their elasticity. Thus the lower the elasticity, the more brittle the material and the more likely it is to break up when subject to prolonged vibration at acoustic resonance. Unfortunately, most piezoelectric materials are very brittle over a wide temperature range.
With respect to (2), piezoelectric generators of radial mechanical vibrations can have Q-factors in the range from 10 to 100. However, when such generators are adapted to compensate for the brittle nature of piezoelectric materials, the Q-factors are typically limited. For example, the use of restraining hoops typically causes the Q-factor of piezoelectric generators to not exceed 10. As a result, piezoelectric motors incorporating such generators generally are limited to slow speeds (a few revolutions per minute) and angular resolutions down to a few angular minutes. In another example, use of a complementary metal resonator generally limits Q-factor to values less than 100. Although, metal resonators, when vibrated, can provide a Q-factor in excess of 100, the piezoelectric elements typically available cannot vibrate such resonators effectively. As a result, any piezoelectric motor based on this type of generator generally exhibits angular speeds of no more than tens of revolutions per minute and angular resolution of no less than one angular minute.